Cosmic dust - what is it and how can we see it?!
Image credit: ESA/Hubble & NASA, R. Sahai, source here.
Did you know space isn’t empty?
Fun fact, did you know space isn’t empty? Yeah! It is not this void that we talk about in the movies. Sure, there are regions of empty space, but on cosmological scales, these voids are actually large regions where there is nothing, and for a really really long way. Sure, there's not much in space for our astronauts to see, but we don't have to go that far, until we reach something else, such as an asteroid belt or meteorite. Normal space, such as interstellar (between stars) and intergalactic (such as between galaxies) space is actually full of gas, dust and radiation!
Dust? I have some of that in my bedroom!
No, no no, not that type of dust! When astrophysicists say dust, they don’t mean the stuff you find when hoovering your room. I’m talking about cosmic dust particles that measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are called meteoroids. Interstellar dust grains have their origin in the material ejected by stars. So once a star has reached the end of its lifetime, it explodes. The material after explosion, such as the atmosphere of a red giant star, is a really nice, cool environment for dust grains to form. After they form, they are thrown into the interstellar medium by radiation pressure, stellar winds or in material thrown off in stellar explosions [1]. They generally start off as carbon or silicate grains, which later accumulate additional atoms of the most abundant elements (hydrogen, oxygen, carbon, nitrogen) to form icy mantles of water ice, methane, carbon monoxide, and ammonia. Here is a diagram and image showing what a cosmic dust grain might look like.
Left: Model of an Interstellar Dust Grain [3]. Right: A microscope view of a dust grain [5]. A typical interstellar grain is thought to consist of a core of rocky material (silicates) or graphite, surrounded by a mantle of ices. Typical grain sizes are 10–8 to 10–7 meters. (This is from 1/100 to 1/10 of a micron; by contrast, human hair is about 10–200 microns wide.) Image and wording from [3].
Dust can be found in many different regions of space, and we name it depending on it’s astronomical location: intergalactic dust (dust between galaxies), interstellar dust (dust between stars), interplanetary dust (dust between planets, such as in the zodiacal cloud) and circumplanetary dust (in the region surrounding a planet itself, or a planetary ring).
What does this mean for astronomy?
Cool molecular gas clouds are a great environment for stars to form. Due to the cool nature, the gas and dust begins to call inwards due to gravity, creating a denser region for star formation to begin. When this new star starts to shine, its energetic radiation which is emitted, can ionise the clouds hydrogen and create a hot bubble of ionised gas. This hot bubble of gas is what we see in this photo. Located within this bubble of hot gas around a nearby massive star are the Free-floating Evaporating Gaseous Globules, or frEGGs for short. These frEGGs are dark compact globules of dust and gas, some of which are regions forming new low-mass stars. The glowing purple edges in the photo above, are the boundary between the frEGG and the hot gas bubble [2]. But, these photos are not what we might see if we looked with our naked eye. Dust is solid, and actually stands in the way of us seeing into space.
Us humans can only see in the visible light spectrum. So when you see beautiful photos of nebula, you're probably seeing it in the infrared spectrum. If we looked in the visible part of the spectrum, we might see something like this, the famous Barnard 68 globule, as the dust blocks the visible light from reaching us, the rest of the stars are literally blocked off:
Image Credit: Bok globule Barnard 68, FORS Team, 8.2-meter VLT Antu, ESO
These dark clouds are dark because they contain myriads of submicron-sized solid particles - the interstellar dust grains we spoke about, and are responsible for the obscuration of light at visible wavelengths. This is also what astronomers call "dust extinction".
So how do we see space and stuff if we have all this dust blocking everything?
Dust clouds are too cold to radiate a measurable amount of energy in the visible part of the spectrum, but because of their small size, they are really good at absorbing ultraviolet and blue light. Once they absorb this radiation, the grains are heated, typically to temperatures from 10 to about 500 K. They then re-radiate this hear at infrared wavelengths, causing dust particles to glow brightly in the infrared [3].
So, if we look at the same region using different wavelengths (in the infrared spectrum) we start to see through the dust cloud. μm means micrometer (which is 0.000001 (10^-6) of a meter)
Image Credit: ESO, FORS Team, 8.2-meter VLT Antu, ESO.
Here, we see the Barnard 68 imaged in six different wavebands, clockwise from the blue to the near-infrared spectral region. From these images we can see that as we increase wavelength, the blocking of the stars from the cloud diminishes, and the cloud size seems to decrease, making the stars more visible [4]. The extinction due to dust is not equally effective at all wavelengths. The shorter the wavelength, the higher the extinction! This means that blue light is affected more strongly than red light. As a result, the stars behind a lot of dust look redder than they really are. This is called interstellar reddening. And although using infrared wavebands helps us see through these stars, we have to be aware that the stars are red due to this effect, and not for some other reason!
Combining images like this with optical images, we see something like this:
Image Credit: ESO, FORS Team, 8.2-meter VLT Antu, ESO.
Our stars are back!! Albeit, a little redder.
Dust is a really important part of astronomy and was once seen as a nuisance, just getting in the way of our pretty photos. But dust is actually really important because we find lots of it around young stars. It is understood that dust can actually unlock a lot of unknown astronomy secrets and mysteries, such as star formation, and seeing as it is also the raw material from which planets like the Earth are formed, planetary and solar system formation!
Sources:
[1] COSMOS, The SAO Encyclopedia of Astronomy, D. Swinburne University of Technology. "Dust Grain". [online]. Available at https://astronomy.swin.edu.au/cosmos/D/Dust+Grain . Last Accessed 8.11.20.
[2] NASA. "Hubble Snaps a Special Stellar Nursery". [online]. Available at https://www.nasa.gov/image-feature/goddard/2020/hubble-snaps-a-special-stellar-nursery/ Last Accessed 8.11.20.
[3] Lumen, "Cosmic Dust". [online]. Available at https://courses.lumenlearning.com/astronomy/chapter/cosmic-dust/ . Last Accessed 8.11.20.
[4] ESO, VLT, "The dark cloud B68 at different wavelengths" . [online]. Available at https://www.eso.org/public/images/eso9934b/ . Last Accessed 8.11.20.
[5] Cosmic dust . [online]. Available at https://herscheltelescope.org.uk/science/infrared/dust/ . Last Accessed 8.11.20.